It is a piece of fabric about 13x13 hanging from the waist covering the butt (the front/sides)sometimes with d-rings, usually plaid or pinstripe, mostly worn by the punk community.
Girl 1: Woah, that bum flap goes great with his bondage pants
Girl 2: ... no bitch.
a bum flap is wen a girl has a really really short skirt, thats rolled over so many times, its like a really thick belt, and so instead of being a skirt .. is a bumflap! (you see the link...), i.e. nothings left to the imagination, especially on a windy day...bumflaps are very common in secondary schools with the *were not even gonna hide were desperate for a shag* kinda girls
"Oo Gladys look at that bum flap...the youth of today!"
"I dont know about that.. i quite like it!"